Fixed bug where the random seed wasn't passed through to
theory_arith. Thanks to Carsten! (Stackoverflow #24327987)

Signed-off-by: Christoph M. Wintersteiger <cwinter@microsoft.com>
Disabled construction of partial model on theory failure as it caused buggy behavior.
Thanks to parno (Codeplex Issue #117)!

Signed-off-by: Christoph M. Wintersteiger <cwinter@microsoft.com>